Frequency modulation detector



Sept. 4, 1956 D. W. BLANcHl-:R

FREQUENCY MoDULATIoN DETECTOR Original Filed Jan. 25, 1950 ATTORN EY United States Patent G 2,761,969 FREQUENCY MGDULA''IN DETECTOR Donald W: Blanchet, North Hollywood, Calif., assigner to Bendrx Aviation Corporation, South Bend, Ind., a vcorporation of Delaware Original application January 23, 1950, Serial No. 140,128, now Patent No. 2,667,626, dated January 26, 1954. Dlvlded and this application March 19, 1951, Serial No.216,431

3 Claims. (Cl. Z50-27) This invention relates to electrical circuits for receiving and detecting frequency modulated waves. The present application is a division of my application Serial No. 140,128, iled January 23, 1950, now Patent No. 2,667,626, issued January 26, 1954, for a Telemetering System for Wells, and the detecting circuit herein disclosed is particularly adapted for although not limited to telemetering with frequency modulated waves.

An object of the invention is to provide an eflicient and simple frequency modulation detector capable of handling low modulation frequencies with accuracy and unaifected by amplitude variations in the received Wave.

Other more specific objects and features of the invention will appear from the description to follow, with reference to the drawing, in which:

Fig. 1 is a schematic diagram of a receiving and detecting circuit incorporating the invention;

Fig. 2 is a set of curves illustrating the operation of the circuit of Fig. l; and

Fig. 3 is a graph illustrating the operation of the frequency discriminators in the circuit of Fig. l.

The circuit of Fig. l is intended to receive a composite signal consisting of two frequency modulated Waves, one in the 8 to 10 kc. band, and the other in the 47 to 49 kc. band over a circuit connected to input terminals A.

A graphical representation of the composite wave form is shown by curve A in Fig. 2. The composite wave A is applied to two band pass amplifiers 110 and 114m. The band pass amplifier 110 will pass only the low frcquency component which may vary in frequency from 8 to 10 kc. The other band pass amplifier l10n will pass only the high frequency component that may Vary in fre quency from 47 to 49 kc. At points B and Ba the resultant separated waves are shown in curves B and Ba. In practice the waves are approximately sinusoidal and the wave B is shown as increasing in frequency with time, Whereas the Wave Ba is shown as decreasing in frequency with time. From amplifier 11i), the varying frequency wave B is passed into a converter 111 whose local oscillator frequency is so chosen as to produce a 200 cycle per'second beat note at the lowest frequency (8 kc.) to be used. The converter also contains a low pass lter to enable a frequency varying from 200 cycles per second to 2200 cycles per second to be passed with negligible attenuation. lThe wave at point C is approximately sinusoidal, varying in frequency from 200 to 2200 cycles per second, depending upon the frequency of the received signal, and is illustrated by curve C in Fig. 2 which is, along with the other curves, exaggerated to illustrate the point. Wave C is fed into a clipper 131 consisting of a condenser 114 in series with resistors 113 and 115 and a tube 112 the control grid of which is connected to the juncture of resistors 113 and 115. On positive excursions, the grid draws current, producing a voltage drop across resistor 113 that limits its potential and clips the positive half waves. On negative excursions, the grid potential is swung past the cut-oli point of the tube, vso that the wave at point D is substantially square topped, and of constant amplitude as illustrated by curve D in Fig. 2. The condenser 114 is of such size as to pass all frequencies within the band 200-2200 cycles with negligible distortion. A load resistor 116 in the anode circuit of the tube 112 is chosen relatively large to accentuate the clipping action.

The square-topped pulses of curve D vary in width inversely with the frequency. They are shaped into pulses of constant width independent of the frequency by differentiating the wave form of curve D in a dilerentiat-l ing network 132, and shaping it in a shaping amplifier stage 133.

The differentiating network 132 comprises a condenser 117 in series with a resistor 118, the junction of which is connected to the control grid of a vacuum tube 119 in the shaping amplifier stage 133. The time constant of the RC circuit consisting of the condenser 117 and the* resistor 11S is small compared to the time duration of the shortest pulses (curve D) applied thereto. The potential developed at the juncture of the condenser 117 and the resistor 11S is represented by curve E of Fig. 2. lt will be observed that this curve consists of a train of alternately positive and negative pips, the amplitude of which is large as compared to the positive and negative potentials required to drive the tube 119. Thus, as shown by the dotted lines in curve E, the positive pips drive the tube 119 well beyond saturation, thereby causing severe clipping in the plate circuit. The negative pips drive the tube 119 well beyond the cut-oli level of thev tube, resulting in severe clipping of the negative pips. The resultant wave in the output circuit of the tube 119 is shaped as shown in curve F which consists of a train of pulses of constant amplitude and constant Width, regardless of Variations in the frequency, the latter simply affecting the distance between successive pips. In curve F the positive pips are of greater magnitude than the negative pips because of the zero bias point selected for the grid of vacuum tube 119 to operate about. However, as will appear later, the relative magnitudes of the positive and negative pips is immaterial, only the positiverpips producing an effect on the linal current.

The anode load resistor 120 of tube 119 is of such size as to accentuate the steep sides of the pulses.

The output of the Shaper amplifier stage 133 at pointl'` is coupled to the input of a power-amplifying frequency discriminating and rectifying stage 134, which produces a direct current the amplitude of which varies with the frequency of recurrence of the positive pips in curve F. Stage 134 includes vacuum tube 121 having its anode connected directly to the B supply, and having an output inductance element 124 in its cathode circuit, so that it functions as a cathode follower. The control grid'of the tube 121 is connected to the juncture of a series connected condenser 123 and resistor 122 to which the pips of curve F are applied from the preceding stage-133. The condenser 123 and resistor 122 are of such value as to provide grid leak bias of the grid of tube 121 by virtue of rectified grid current. The output inductance element 124 is shunted by a rectifier 140 so poled as to bypass negative impulses from the cathode of tube 121 past the inductance element 124. On the other hand, positive pulses applied from the cathode to the upper end of the inductance element 124 are conducted through a rectifier 141 to a lter consisting of a series inductance element 126, -a shunt condenser 127, a series resistor 12S, and a potentiometer 129. The movable Contact of the potentiometer 129 is connected to one output terminal, and the lower end of the potentiometer 129 is connected by a biasing battery 130 to the other output terminal.

The action of stage 134 will be explained with reference to the diagram of Fig. 3 which shows a curve 143 Patented Sept. 4, 1956 representing the variation of cathode current in the tube 121 with variations in grid voltage, the latter being plotted horizontally, and the cathode current vertically. The pips of curve F are applied to the grid of the tube, and a train of such pips is shown as including positive rpips y144 and 145 equally spaced apart, and pips 146, 147, 14S, 149, 150, 151 and 152 spaced progressively closer together. Corresponding intervening negative pips are identiied by the same reference numerals with the suffix a.

As has been previously explained, all of the pips applied to the stage 134 are of'equal magnitude, but differ only in their spacing. Each positive pip applied to the grid of tube 121 causes the latter todraw current and charge theV condenser 123, thereby applying a negative bias to the grid. This negative bias gradually leaks oft through the resistor 122, but so long as the positive pips applied to the tube are equally spaced apart, the average bias of the grid will remain constant. However, if the pips come at closer intervals, the negative bias on the grid will increase, and its value will be a measure of the `frequency of recurrence of the pips. Thus, referring to Fig. 3,*let

, it be assumed that the pips 144v andy 145 are uniformly spaced apartthe same as the preceding pips, and that this spacing was such as to produce a grid bias represented by the upper end portion of the dotted line 160. The positive pip 144 will drive the tube beyond saturation, and produce a current pip 14419 in thefcathode circuit of the tube 121. The succeeding negative pip 144a applied to the grid of the tube reduces'the conductance of the tube and produces a negative current pip 144e` in the cathode circuit. The positive and negative pips in the output circuit of the tube remain constant so long as successive pips applied to the input circuit are Vuniformly spaced. However, it will be noticed that .the positive pip 146 in Fig. 3 follows the pip 145 more closely than the latter followed pip 144, this indicating an increase in frequency of the original wave. Because of the reduced lapse of time, less of the charge imposed on the condenser 123 can vleak off between pulses, and the grid bias as represented by the dotted line 160 in Fig. 3 increases to a more negative value. The result is that the positive current pip 146b in the output circuit starts from a lower base and is therefore larger. The corresponding negative pips become shorter, but this is of no moment because the negative pipsare blocked -by the rectiler 141 and shorted across indue-tance 124 by rectifier 140 and do not appear inthe iinal output.

`ln Fig. 3 Vthe pips from 146 to 152 inclusive recur at increasingly more frequent intervals, so that the potential on the grid of tube 121 becomes progressively more negative, as shown by line 160, the output positive current pips become progressively larger, and the unused negative pips Abecome progressively smaller.

Referring to curve G of Fig. 2 which shows the potential developed across the inductance element 124, it will be observed that the pulses (positive) developed across this inductance element is response to the positive pulsesV applied to the grid not only recur at shorter intervals with increasing frequency, but are also of increasing magnitude. When integrated by the filter, consisting of the elements 126, 127 and 128, they become a direct current of increasing magnitude yas shown in curve H.

The circuit associated with the tube 121 has several unique characteristics. In the rst place, whereas it is customaryV to employ a resistor as the output impedance element of Ia cathode follower pulse counter, the present circuit employs the inductance element 124. This markedly improves the eiciency of the circuit because the Vinductance element has a very low resistance and hence the average bias potential of the cathode is subcreasing negati-ve bias developed on the grid of the tube,

v and would materially reduce the output obtainable with forms the function of discarding the negative pips' developed in the output circuit of the tube 121. As shown in Fig. 3, these pip`s.(144c-151c) vary in amplitude inversely with respect to variations in amplitude of the positive pips (144b-152b) and would tend to neutralize the latter. It is therefore an essential element of the circuit.

The rectifier 14d is desirable because it prevents the development of negative Yvoltage pips resulting `from decay of positive current pips in inductance elementf124. Another action of the rectifier 144B is to bypass the negative current pips around the inductance 124 Vto prevent a positive surge from being formed by the decay of the negative surges which would tend Vto counteract the action of the desired positive pips across inductance 124. The biasing battery 131i makes possible the delivery,

to the output terminals of a direct Vcurrent varying'strictly in accordance with the frequency variationsV received. it has been previously indicated that the lowest frequency employed is 200 cycles per second, andfthis would represent a zero value of data. transmitted. Since a frequency of 200 cycles per second would produce a cur- The time constant of the CR circuit consisting of theV condenser 123 and the resistor 122 is large compared to the time interval between the pulses applied thereto atV the minimum frequency of 200 cycles per second, but it must not be so large as to damp out the most rapid frequency variations that may occur. For the telemetering function described, the circuit must pass a maximum pulse recurrence rate of 50 cycles per second.

The lower channel in Fig. 1 beyond the frequency converting unit 111e is identical with the upper channel, the corresponding elements being represented by blocks bearing the same reference numerals with the suiiix a. In Fig. 2 the curves Ca to Ha lare shown as decreasing in frequency instead of increasing. Otherwise they are similar to curves C to H. They would be the same as curves C to H if similar frequency modulations were applied to both the l0 kc. and the 50 kc. carriers in the well. i

The output currents of the units 134 and 134:1 may be applied to two elements of a recording oscillograph.

Although for the purpose of explaining the invention,

a particular embodiment thereof has been shown and described, obvious modifications will occur to a person skilledy in the art, and I do not desire to be limited to the exact details shown and described.

I claim:

1. Apparatus comprising, in combination with a source of recurrent positive pulses of constant amplitude and width but of variable repetition rate to be detected: a cathode follower tube having an anode, cathode `and co1- trol grid; a source of anode potential having a positive terminal connected to said anode, and having a negative terminal; an inductance element directly connecting said cathode to said negative terminal; 'an input condenser connecting said source of positive pulses to .said grid, and a grid leak resistor connecting said grid -to said negaytive terminal; the ohmic resistance of said inductance 5 means for applying to said filter means positive pulses developed at said cathode; said positive pulses supplied by said source being of such magnitude relative to the characteristic of the tube as to saturate the tube.

2. Apparatus according to claim 1 in which said rectifying means comprises a half Wave rectifier connected in series relation between said cathode and said lter means.

3. Apparatus according to claim 2 including a second half wave reetier in shunt to said inductance and so poled as to bypass negative pulses from said cathode or negative pulses generated by the decay of the usable positive pulses.

References Cited in the le of this patent UNITED STATES PATENTS Barkley Feb. 14, 1933 De Rosa .lune 20, 1944 Scott Nov. 14, 1944 De Rosa May 25, 1948 Gray May 25, 1948 Hopper June22, 1948 Forster Sept. 12, 1950 OTHER REFERENCES Cooking: Modern Detector Circuits in The Wireless World, August 17, 1939. 

